1. Field of the Invention
This invention relates to a stator, a spindle motor, and a recording disk driving apparatus, or in particular to a thin spindle motor.
2. Description of the Related Art
With the recent improvement in the storage capacity density of the hard disk, demand has increased more and more for a smaller hard disk drive. Further, applications to the portable devices have spread. The portable devices are often used in an environment liable to be subjected to vibrations and shocks, and therefore required to have a high durability against external forces. An increased durability against external forces requires an increased rigidity of the bearing, resulting in an increased bearing friction loss. On the other hand, a power supply such as a dry cell or a storage battery not large in power capacity is often used for the portable devices, and a reduced power consumption is also required.
FIG. 7 is a longitudinal sectional view showing the structure of a conventional spindle motor. The spindle motor 100 of FIG. 7 classified as a brushless motor includes a fixed assembly 23 having a stator 131, a rotor 21 having a rotor magnet 132 and a bearing 22.
The stator 131 includes a plurality of teeth, a core back magnetically connected to the outer periphery or the inner periphery of the teeth and a coil wound on each of the teeth. The teeth and the core back are formed by stacking a plurality of core sheets molded from a silicon steel sheet high in permeability. The annular rotor magnet 132 is opposed radially to the teeth.
FIG. 6 shows another example of a conventional spindle motor. In the spindle motor shown in FIG. 6, the end portion of the uppermost core sheet 134a opposed to the rotor magnet is bent axially upward. With this structure, the teeth radially opposed to the rotor magnet 132 have so large an area that the magnetic fluxes of the rotor magnet 132 can be efficiently utilized. In other words, a large torque constant is obtained. Even in the case where the current flowing in the coil 131a is reduced, therefore, a torque equivalent to the current not reduced in the structure with the core sheet not bent is obtained, thereby making it possible to reduce the power consumption.
Nevertheless, several problems are encountered to further reduce the thickness of this spindle motor 100.
The silicon steel sheet finding wide application as a core sheet is often formed by pressure rolling and has its own limit of reducing the thickness. To reduce the thickness of the spindle motor, therefore, the number of the core sheets stacked is required to be reduced.
The magnitude of displacement from the ideal angle when the single core sheet 134a is bent, i.e. the bending margin (D1 in FIG. 6) is in the range of about 0.5 to 1.5 times the thickness (D2 of FIG. 6) of a single core sheet.
In the spindle motor shown in FIG. 6, the bending margin D1 of the core sheet 134 may happen to be radially opposed to the portion of the rotor magnet 132 having a large magnetic flux density.
In the case of a motor having an axial thickness of less than 10 mm, on the other hand, about two to five core sheets are stacked, and therefore, the axial height of the bending margin of the core sheet represents a proportion not negligible of the axial height of the rotor magnet.
In the bending margin of the core sheet, the radial interval between the magnet and the plurality of the teeth is larger than the ideal radial interval between the rotor magnet and the stator. The force generated by the magnetic interaction is decreased in inverse proportion to the square of distance. Even in the case where the core sheet is bent, therefore, the torque constant cannot be sufficiently improved.
With the decrease in thickness, the core sheet cannot be bent by a sufficient length and therefore it is difficult to secure the length of the bent portion.
Further, in the case where the inner peripheral portion of the teeth is processed after stacking a plurality of core sheets, the interval between the stacked core sheets may be widened, often resulting in an uneven thickness of the stator core. Thus, the axial attraction force of the stator with the motor driven is changed, with the result that the electromagnetic noises are occasionally increased or the bearing performance reduced. This trend is especially conspicuous with a thin spindle motor. Also, in the case where the spindle motor is mounted on a portable device, the noises generated are unpleasant to the user, while the lower bearing performance reduces the driving efficiency and hampers the longer life of the motor.